An important measure of the remaining discharge time period t during which a cell can deliver power to an electrical load is the cell's remaining capacity ("reserve capacity") at a constant discharging current I.sub.d. Typically, this remaining capacity is measured in ampere-hours (AH). In turn, it is important to have a remotely located sensing means (connected by wires to the cell) for determining this time period t=AH/I.sub.d.
Lead-acid cells generally have the property that, except for a relatively small time interval t.sub.o (approximately between 0.2 t and 0.3 t) toward the end of this time period t, the terminal voltage V of the cell is approximately constant. Thus the power P=VI.sub.d delivered to the load will be approximately constant except during the relatively small time interval t.sub.o.
In a paper entitled "Internal Resistance: Harbinger of Capacity Loss in Starved Electrolyte Sealed Lead Acid Batteries," published in Intelec 87 Conference Proceedings, pp. 128-131 (1987), and authored by F. J. Vaccaro and P. Casson, a technique for determining the remaining capacity of a valve-regulated lead-acid cell is disclosed. In that technique an alternating voltage at 1 KHz is applied across the terminals of the cell and the impedance is measured and compared with a calibration curve. However, the technique essentially measures only the purely resistive component of the impedance of the electrolyte in the cell and therefore other relevant parameters--which depend on the history of the cell's charging and discharging, heating and cooling, etc.--cannot be detected. Thus, the only detectable failure mechanisms are loss of water--other failure mechanisms relevant to remaining capacity are not detected. In particular, the technique does not detect aging, or flaking of electrodes, or insufficient organic additive, or irreversible formation of lead sulfate, all of which depend upon the history of the cell and which deteriorate the condition of the cell, and thus reduce the actually remaining capacity of the cell, but undesirably are not detected by the technique.
In U.S. Pat. No. 4,952,862 a technique for predicting the remaining discharge time of a lead-acid storage cell for a given current is disclosed. That technique, however, requires a significant discharge of the cell to make a measurement. Such a discharge can be undesirable because of the consequent deterioration of the cell simply as a result of this required significant discharge.
It would therefore be desirable to have a technique for measuring the remaining capacity of a storage cell that mitigates the shortcomings of prior art.